This invention generally relates to the field of humidity measuring devices and, more particularly, is directed to a humidity-sensitive resistance element for use in measuring humidity.
There are many devices known in the prior art for measuring the amount of humidity in the atmosphere. Many of these devices employ a humidity-sensitive element which varies in resistance depending on the amount of humidity exposed to the element. Thus, the element may be connected in an electrical circuit to provide an output signal representative of the amount of humidity present in the environment.
Most of the humidity measuring devices which employ a humidity-sensitive resistive element measure the amount of humidity by detecting a change in resistance value of the element. The resistance value of the element changes as a result of moisture physically adhering to the surface of the body of the element. The element body is usually made of a polymer, a sintered metal oxide or is formed from a metal oxide layer sintered on a substrate. Under low humidity conditions, the resistance value of such an element is very large, e.g., in the range 1 to 10 M.OMEGA.. Thus, elements of this type cannot be used in electrical circuits without the addition of impedance matching means. Moreover, the degree of accuracy of such an element decreases with prolonged use. The decrease in accuracy is due to the surface of the element becoming contaminated over time, thus not allowing moisture to adhere to the body of the element.
A humidity-sensitive resistive element having a body made of metal oxide is disclosed in U.S. Pat. No. 4,080,564 issued to Nitta et al. Nitta discloses that the long-term accuracy of such an element can be improved by applying heat to the element before each use. The heat is used to clear away any contaminates which may be present on the surface of the element. The requirement that the element be heated before each use means that continuous monitoring of humidity is impossible. Furthermore, the addition of a heater and heating circuitry adds to the cost of the humidity measuring equipment.
Japanese Patent Disclosure No. 56-4204, published Jan. 17, 1981, discloses another humidity-sensitive resistive element. The element comprises 85 to 99 mol% of ZnO, 0.5 to 10 mol% of LiZnVO.sub.4, and 0.5 to 5 mol% of Cr.sub.2 O.sub.3 +Fe.sub.2 O.sub.3. The element has somewhat stable characteristics when it is used over a long period of time at relatively low temperatures and in normal environmental conditions (as for example, 0.degree. to 40.degree. C. at 30 to 90% relative humidity). The element may thus be used in air conditioners and humidifiers. The element, however, may not be used in high temperature environments as the accuracy of the element is adversely affected.
Another type of humidity-sensitive resistive element is described in U.S. Pat. No. 3,926,858 issued to Ichinose et al., and comprises 89.9 to 20 mol% of ZnO, 0.1 to 20 mol% of Cr.sub.2 O.sub.3 and 10 to 60 mol% of at least one member selected from the group consisting of a third metal oxide. It is disclosed that when the content of Cr.sub.2 O.sub.3 is more than 20 mol%, the resultant element has a resistivity of over 100 M.OMEGA. and is, therefore, unsuitable as a moisture-sensitive element. While the resistance range of the humidity-sensitive element disclosed by Ichinose is lower than some elements known in the art, it it still very high and must be used with an impedance matching network.
The instant invention overcomes the above disadvantages of humidity-sensitive resistive elements known in the prior art by a novel selection of the semiconductor materials which forms the element. In particular, the present invention produces a humidity-sensitive element having a relatively low electrical resistance range and which exhibits good stability, even after prolonged use at high temperatures. The element does not have to be heated before each use in order to maintain its accuracy. Thus, continuous monitoring of humidity is possible. The element also does not require an impedance matching network and may be directly connected into an electrical circuit. The element may also be used over a wide range of humidity conditions and over a wide range of temperatures.